Develop of A Torque Feedback Interface for Virtual Acupuncture
Training
08118118, Shuang Peng, School of Automation
1. About
Haptic devices provide a straightforward method for mapping simulated forces to the user. This
enables the user to actively interact with the virtual environment. This makes them useful in
applications like medical training, where users can operate with virtual tissue before moving to real-
world patients. Acupuncture is a medical technique that inserts thin needles through patient’s skin
at body’s specific points. This report presents the development of a lightweight, highly backdrivable
torque feedback tip for virtual acupuncture training. The tip would be adapted to an existing
Phantom device.
2. Hardware
The acupuncture process mainly involves two motions, insertion of the needle and rotation in the
tissue. So the haptic device needs to render insert force and rotation torque in order to simulate the
acupuncture process on real-world patients. However, most existing commercial haptic devices are
equipped with 3 active translation DoF and 3 passive rotation DoF. Such as 3D systems’ Phantom[1],
Falcon[2], and Omega[3]. For those with additional active rotation feedback hardware, motors and
transmission systems are directly mounted on the end-effector of the device. As a result, additional
weight and inertia can impact apparently on user’s experience. To address this issue, some solutions
compensated the tip’s weight on the software level with device’s dynamic model[4]. This project
proposed a tendon-based flex transmission structure that allows the tip’s actuation motor to be
decoupling mounted on device’s base. Which greatly decreases the tip’s moving weight. At the same
time, an origami-inspired clutch mechanism is designed to reduce the friction of free rotation. The
prototype of the tip is shown in Fig 1.
Fig 1 Prototype of the torque feedback tip
The design of the tip follows these guidelines.
Weight – Weight of the tip must be limited, without cause obvious changing on device's dynamic
property. Also, it should be easy and comfortable to operate by users.
Workspace – The added tip should not interfere device’s original workspace.
Transparency – Tip’s actuation system should be low friction, low inertia, and highly back-drivable,
which renders the feedback torque directly to users.
Control and sensing – The tip should be torque-controllable with an encoder measuring needle’s
real-time position and angular velocity.
A. Flex transmission system
To decouple the actuation motor from the tip, this flex transmission system is designed to
transparently transfer feedback torque from a fixed base to moving tip. The core of the system is a
set of capstans linked by tendons, which converts torque to the tendon’s tensile force. The tensile
force is then transmitted via a pair of tendons sliding inside a flexible Teflon tube. Nylon tendon
and Teflon tube are selected to minimize the sliding friction. CAD drawings of the system are shown
in Fig 2 and Fig 3.
Fig 2 CAD drawing of the moving tip
Fig 3 CAD drawing of the fixed motor base
B. Rotation clutch
The diameter of a common acupuncture needle ranges from 1mm to 3mm. Smaller diameter means
users’ fingertip skin is easier to deform when twisting the needle. As a result, rotation friction is
easier to be perceived by users. To reduce the free rotation friction, a clutch mechanism is developed
to disconnect the needle when there’s no feedback torque, shown in Fig4. The clutch consists of 3
pairs of symmetrical claws parallel linked to a central platform. When the central platform is
actuated, claws retract inward to lock a high friction output shaft, which connects directly to the
needle. Under retract state, the clutch is able to bypass torque up to 0.4 Nm.
Fig 4 The clutch
Since the clutch is mounted on the tip, an origami-inspired design is adapted to reduce the
mechanism’s weight. The finished structure weights only 2.1g, shown in Fig 5.
Fig 5 Manufacture process of the clutch
C. Actuation motor
Actuation motor is the defining factor of haptic systems’ performance. Most existing applicants use
coreless DC motors with planetary gear reducer. However, geared reducer would introduce
unwanted friction to the system. The actuators used in this project are 1613 BLDC motors with a
KV value of 130, which provides continuous torque up to 0.08Nm under direct drive configuration.
Although this torque is lower compared to most geared coreless motors, they’re adequate for
generating rotation feedback with small diameter needles. Also, the output torque of the BLDC
motor has a good linear relationship with the phase current. This enables precise close-loop torque
control with current sensing embedded in the controller. Fig 6 shows the motor used in this project.
Fig 6 Motor module
D. Encoder
In acupuncture simulations, the needle’s position and velocity must be precisely measured in order
to calculate simulated feedback torque. Both tip’s rotation shaft and actuator motor are equipped
with magnetic encoders shown in Fig 7. Encoders are AMS5600 from austriamicrosystems AG with
a resolution up to 0.088Β°.
Fig 7 Encoder
E. Controller
A customed controller board is designed as shown in Fig 8. The board contains 2 BLDC motor
drivers with phase current sampling, 2 IIC ports to read encoders, and a CAN bus to communicate
with the host computer. A dedicated control loop running at 1.5kHz sends measured needle position
and velocity to the simulation, then receives calculated feedback torque to actuate the clutch and
needle.
Fig 8 Controller
F. Feedback torque algorithm
With the pose of tip, the insertion depth of virtual needle in tissue can be obtained, shown in Fig 9.
Fig 9 Virtual needle inserted in tissue
The target feedback torque can then be calculated by the following algorithm.
𝜏 = βˆ‘(π‘Žπ‘–βˆ™πœƒ + π‘π‘–βˆ™πœƒσ°‡—)βˆ™πΏπ‘–
Where 𝜏 is the calculated feedback torque, π‘Žπ‘– is virtual tissue’s elastic coefficient, 𝑏𝑖 is virtual
tissue’s damping coefficient, πœƒ is needle’s position, and πœƒσ°‡— is needle’s velocity. 𝐿𝑖 are needle’s
stagnations in each layers of tissue. By tuning π‘Žπ‘– and 𝑏𝑖, it should be possible to simulate the
general texture of different tissues, such as skin, muscle, etc.
3. Preliminary evaluation of the system
An experiment has been designed to test the tip’s torque feedback capability. During the experiment,
the tip is controlled to render a 0.02Nm step and sine torque output. Torque is measured by a torque
sensor shown in Fig 10.
Fig 10 Test bench setup
The measured result is shown in Fig 11. From the plot, it can be seen that the output torque is
basically useable. The noise on the signal can be improved via fine-tuning of the motor controller’s
paraments.
Fig 11 0.02Nm step output and sine output
4. Conclusion
This report presented the development of a lightweight, highly backdrivable torque feedback tip for
virtual acupuncture training. The proposed design includes a flex transmission system that
minimizes tip’s weight and an origami-inspired clutch to reduce free rotation friction. At the same
time, a compact controller is developed to control BLDC actuation motors. With a designed
evaluation, the system is able to deliver desired feedback torque calculated by the host computer.
The future work will concentrate on the feedback control of the clutch mechanism through a new
design with a pressure sensor. This will allow applications where controllable dumping modules are
needed.
References
[1] https://www.3dsystems.com/haptics-devices/3d-systems-phantom-premium
[2] https://hapticshouse.com/pages/novints-falcon-haptic-device
[3] https://www.forcedimension.com/products/omega
[4] Karbasizadeh, N., Zarei, M., Aflakian, A., Masouleh, M. T., & Kalhor, A. (2018). Experimental
dynamic identification and model feed-forward control of Novint Falcon haptic
device. Mechatronics, 51, 19-30.